A chain can be too tight Jeanette, but as long as there is a little vertical free play a fairly new bike like yours will be alright. Once a chain is well worn, it can develop tight spots due to wear being uneven. In other words, there can be enough slack at one point of rotation, but tightening too much at another, so adjustment has to be for the tightest point to have some slack.
A loose chain won't increase drag unless it's nearly falling off or very worn. Electric assist bikes are generally difficult to pedal without power, only the Twist series being reasonable in this respect. Those with hub motors like your Sprint do have some drag due to the hub. This isn't noticeable when you push the bike at walking pace, but at cycling speeds it increases. There is an internal freewheel which disconnects the motor when pedalling without power, but the rotating wheel is still having to turn the orbital gears in the hub, and it's those and their grease that give rise to the drag.
The other part of the drag comes from the wide tyres and the extra weight of an electric bike bearing down on those. The friction and flexing of the rubber on the road all absorb energy, and it's best to keep the tyres up to full pressure all the time to make pedalling as easy as possible.
When there's any risk of the battery not lasting a trip, I always carry a spare in one pannier to ensure I don't have to struggle without power in my area, hilly like yours.
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